This image shows the synthesis of the lotus-like rough structure of the new superhydrophobic nanomaterial. Image: Shirin Alexander/University of Swansea.
This image shows the synthesis of the lotus-like rough structure of the new superhydrophobic nanomaterial. Image: Shirin Alexander/University of Swansea.

A new class of superhydrophobic nanomaterials might simplify the process of protecting surfaces from water. Developed by scientists at Rice University, the universities of Swansea and Bristol in the UK and the University of Nice Sophia Antipolis in France, the superhydrophobic nanomaterial is inexpensive, nontoxic and can be applied to a variety of surfaces via spray- or spin-coating.

The hydrocarbon-based material could be a ‘green’ replacement for the costly, hazardous fluorocarbons that are commonly used for superhydrophobic applications. "Nature knows how to make these materials and stay environmentally friendly," said Rice chemist Andrew Barron, who led the team of scientists. "Our job has been to figure out how and why, and to emulate that." The scientists report their find in ACS Applied Materials and Interfaces.

The lotus leaf was very much on their minds as the researchers tried to mimic one of the most hydrophobic – water-repelling – surfaces on the planet. Barron said the leaf's abilities spring from its combination of microscopic and nanoscale features.

"In the lotus leaf, these are due to papillae within the epidermis and epicuticular waxes on top," he explained. "In our material, there is a microstructure created by the agglomeration of alumina nanoparticles mimicking the papillae and the hyperbranched organic moieties simulating the effect of the epicuticular waxes."

Fabrication and testing of, what the researchers call, a branched hydrocarbon low-surface energy material (LSEM) were carried out by lead author Shirin Alexander, a research officer at the Energy Safety Research Institute at Swansea University.

Alexander coated easily-synthesized aluminum oxide nanoparticles with modified carboxylic acids that feature highly branched hydrocarbon chains. These spiky chains are the first line of defense against water, making the surface rough. This roughness, a characteristic of hydrophobic materials, traps a layer of air and minimizes contact between the surface and the water droplets, allowing them to roll off.

To be superhydrophobic, a material has to have a water contact angle larger than 150°. The contact angle is the angle at which the surface of the water meets the surface of the material: the greater the beading, the higher the angle. An angle of 0° is basically a puddle, while a maximum angle of 180° defines a sphere just touching the surface.

The LSEM has an observed contact angle of about 155°, essentially equivalent to the best fluorocarbon-based superhydrophobic coatings. Even with varied coating techniques and curing temperatures, the material retained its qualities, the researchers reported.

Potential applications include friction-reducing coatings for marine applications, where there is international agreement for trying to keep water safe from such potentially dangerous additives as fluorocarbons. "The textured surfaces of other superhydrophobic coatings are often damaged and thus reduce the hydrophobic nature," Barron said. "Our material has a more random hierarchical structure that can sustain damage and maintain its effects."

He said the team is now working to improve the material's adhesion to various substrates, as well as looking at large-scale application to surfaces.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.